Question on testing transistor 3904

Discussion in 'General Electronics Chat' started by Rattle49, Sep 29, 2015.

  1. Rattle49

    Thread Starter New Member

    Sep 22, 2015
    Hello Everyone,

    I am testing transistor 3904 with a digital multimeter. When I put it on diode function the test result was perfect. reads 0.66V and 0.63V. But I also want to be able to use resistance function to test. While my results are around 7 or 8 mega ohms. I thought it should be somewhere around 500 ohms. I am just so confused. Please help........
  2. OBW0549

    Distinguished Member

    Mar 2, 2015
    The reason you're not getting the results you expect is that your DMM uses a very low voltage to measure resistance, and this voltage is not enough to forward bias semiconductor junctions and get a meaningful reading. One of my DMMs applies only 400 mV and the other one, only 200 mV. Yours is probably similar.
    cmartinez likes this.
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    If your DMM has a diode test mode as one of its Ohmmeter ranges, then it applies sufficient voltage across the device being measured to forward bias a Si junction. Mine even causes a red LED (Vf=~2V) to glow dimly...
    cmartinez likes this.
  4. Rattle49

    Thread Starter New Member

    Sep 22, 2015
    Ah that makes sense. Thank you so much
  5. dannyf

    Well-Known Member

    Sep 13, 2015
    Why did you think so?

    The "resistance" of a p-n junction is a function of the current through it, or alternatively, the voltage across it. Without knowing either of that, it could be anything from 100s of Mohms down to a few ohm.
  6. dl324

    AAC Fanatic!

    Mar 30, 2015
    Aside from performing a diode test (dead or alive), a multimeter isn't well suited to test any other transistor parameters.
  7. vk6zgo

    Active Member

    Jul 21, 2012
    It won't give you definitive results like a proper tester,but you can check your NPN BJT for gain with a multimeter.

    On "diode test" position,place the red lead on the collector,the black on the emitter.
    Now spit on your finger & place it between the base & collector.
    The meter will show collector current as a drop in resistance.

    This will also work for PNP with the red & black leads transposed.

    I mainly use this as a "go/no-go" test,or to determine which lead is the collector on unknown transistors.
    A reversed transistor will show lower gain than a correctly connected one.
  8. Rattle49

    Thread Starter New Member

    Sep 22, 2015
    I read it from a book saying that if you test your transistor with resistance function, "the DMM should read a low forward resistance, typically less than 1kΩ". the figure in the book just shows a DMM with 1 lead on emitter and the other on base.